This invention relates to a multi-level code transmission system for transmitting multi-level PCM signals with high fidelity and more particularly relates to a novel and improved code transmission system wherein a d.c. restoration can be effected in such a way that levels of the transmitted code signals can be reproduced with high fidelity when receiving the transmitted multi-level PCM signals.
In code communication employing such a code signal as PCM signal, there is a problem of code error due to the frequency characteristic of the transmission line. That is, if the transmission line has a low cut-off frequency characteristic, a phase distortion occurs in the low frequency component of the transmitted signal, so that the level of the transmitted code signal is varied with a relative long period through the transmission line by the low cut-off frequency characteristic thereof, and accordingly there occurs code error at the receiving end.
In conventional PCM communication, PCM regenerative repeaters are inserted at every given distance between the transmitting end and the receiving end so as to reshape the transmitted PCM signal in order that the receiving end can receive the PCM signal having waveforms of high fidelity, i.e., having waveforms quite similar to that of the original signal. The PCM regenerative repeater is as diagrammatically illustrated in FIG. 1 of the drawings. The repeater includes an equalization amplifier 1 for amplifying and equalizing or reshaping the incoming pulse train, a timing extracting circuit 2 having a retiming function which is coupled to said amplifier so as to extract the timing information from the output code pulse series of said amplifier 1 and to form a timing signal relating to said output code signal and which is similar in some respects to that described in applicant's co-pending U.S. application Ser. No. 361,581 now U.S. Pat. No. 3,854,010 filed May 18, 1973, entitled "A Time Division Multiplexing Transmission System" and assigned to the assignee of the present invention, especially by reference to the description from page 65, line 12 to page 66, line 1 and the block 275 in FIG. 23, and a level identifying circuit 3 having a PCM pulse regeneration function, i.e., to which the output signal from said amplifier 1 is applied to be sampled by said timing information as a sampling pulse, in which the level of the sampled signal is identified whether the sampled value exceeds a given value or not so as to regenerate the reshaped pulses which are transmitted toward the receiving end and which is similar in some respects to that described in said U.S. application Ser. No. 361,581, now U.S. Pat. No. 3,854,010 especially by reference to the description from page 48, line 26 to page 50, line 7 and FIG. 18.
In such a communication system utilizing the regenerative repeater, if the transmission line has the low cut-off frequency characteristic, then the average d.c. level of the PCM signal is varied, as mentioned above, and accordingly, the zero level of the d.c. component in the input signal of said level identification circuit 3 is relatively different in point of waveform from that of the signal shape at the transmitting end. Thus, there occurs code error at the output of the level identification circuit 3. In such known devices, in order to eliminate code error, employed are a first method for balancing d.c. components with a code formation having redundancy in the direction of the amplitude of a code signal (for example, by transmitting a four-level code with five levels), a second method for restoring the d.c. component by peak-clamping wherein the code pulses have redundancy in the direction of the time axis so as to insert clamp pulses in the series of said pulses and so on.
Examples of known code formations embodying the first method are the PST (Paired Selected Ternary) code, symmetric or balanced pseudo-ternary code, symmetric or balanced multi-level code, and so on. By using such a code formation, the influence of the low cut-off frequency characteristic of the transmission line can be eliminated, however, the configuration of a decoder circuit at the receiving end is remarkably complicated because of the special code formation. In addition, due to the redundancy in the direction of amplitude of the code signal, this method has a drawback from the standpoint of the S/N ratio necessary for maintaining the code error rate at a given value.
In the case of a signal having a PCM signal only, the d.c. component can be easily balanced by said first method, whereas in the case of a composite signal having a PCM signal and a further signal such as a composite signal having the frame of a picture signal in the form of an analogue signal and the frame of an audio signal affixed to said picture signal in the form of a PCM signal and both of which frames are transmitted alternately, for example in a still picture transmission system described in said U.S. application Ser. No. 361,581, now U.S. Pat. No. 3,854,010 especially by reference to the description from page 23, line 18 to page 24, line 1, the former method cannot be adopted, since the signal level is varied when transmitting the analogue signal.
Therefore, in order to perform the d.c. restoration in such a composite signal, the second method of peak-clamping is employed, in which clamping pulses are inserted in the PCM pulse series so as to clamp the peak of the clamping pulses.
FIG. 2 shows an arrangement of a PCM regenerative repeater of the prior art in which the d.c. component is restored by peak-clamping so as to identify and to reproduce the transmitted original signal. In FIG. 2, a peak clamping circuit 4 is inserted between the preceding equalization amplifier 1 and the following timing extracting circuit 2 and the following level identifying circuit 3. The circuit arrangement of said peak clamping circuit 4 for d.c. restoration is a conventional circuit, as shown in FIG. 3, which comprises a diode D1 and a capacitor C1. In FIG. 3, the signal from the equalization amplifier 1 is applied to an input terminal In connected to the capacitor 3, the other terminal of which is coupled to a reference voltage E.sub.s through the diode D1. An output Out is derived from the common connecting point of the capacitor C1 and the diode D1. If the signal having clamp pulses and applied to said input In exceeds the clamp level corresponding to the fixed voltage E.sub.s, the diode D1 conducts to discharge (or charge) said capacitor C1 with a small time constant so as to restore the d.c. component.
In case that the waveform of the code signal is a NRZ (Non-Return-to-Zero) pulse, and that there provides no level limitation in the pulse series, if the pulse series continues with the pulses without there existing any zero level pulses, then a sag of a relatively long period occurs in the pulse series due to the low cut-off frequency characteristic of the transmission line. In order to prevent the occurrence of said sag, a reference signal representative of a zero level is inserted into the PCM signal at a relatively long period so as to clamp that zero level, as shown in FIG. 4a. In FIG. 4a, a signal a illustrates said reference signal having a zero level at such a period as being able to neglect said sag in the signal waveform. The insertion period of this reference signal a is usually equal to one PCM frame period. In FIG. 4a, other signals b and c illustrate a frame synchronizing signal and a four-level PCM signal, respectively. FIG. 4b shows a clamping pulse used to form said reference signal a.
According to this transmission system, the decoding can be processed easily at the receiving end because a special coding system is not used. However, this transmission system has the following defects. First of all, if the noise n is superimposed on the PCM signal as shown in FIG. 5, the signal is clamped at the peak point a' of the noise component, so that the restored d.c. level a of the code signal is raised by the amplitude of the noise n. This rise in the d.c. level deteriorates the code error rate. Another defect is the possibility of the considerable variation of an average level of the signal series in case there is no level limitation in the signal series. That is, in the peak clamping circuit 4 shown in FIG. 3, the ratio r.sub.b /r.sub.f between the backward resistor r.sub.b of the diode D1 (shown as the dotted line in FIG. 3) and the forward resistor r.sub.f of the diode D1 is finite, so that the restored d.c. level is varied in accordance with the ratio r.sub.b /r.sub. f in the case of the signal waveform having continuous parts of any given level. This variation of the d.c. level deteriorates the code error rate.
As a result, it clearly seems to be advantageous to use a pulse clamping process, for example employed in the processing of the television signal. In the pulse clamping system, the reference level of a PCM signal is clamped in the form of pulse, so that it is necessary to produce the clamping pulse having a period corresponding to that of the reference level. In the case of the television signal, the clamping pulse can easily be produced by separating the synchronizing signal from the television signal by identifying the amplitudes of the television signal. On the contrary, when introducing the pulse clamping process to the PCM signal, there occurs the following contradictory difficulty. Namely, in order to produce the clamping pulse, it is necessary to detect the synchronizing signal firstly, and in order to detect the synchronizing signal, it is required to identify the level of a code signal prior to the detection of the synchronizing pulse. However, in order to derive a pulse having an accurate level required for identifying the code signal, it is necessary to restore a d.c. component of said code signal. Accordingly, if the pulse clamping is adopted to the transmission of a code signal, the relation goes round and round in circles as described above and it is difficult to realize the d.c. restoration only by using the pulse clamping.